The present invention relates to an apparatus for the measurement of a parameter which can be transduced using a resistively varying transducer element. One example of a parameter which is amenable to measurement using the present invention is temperature which may be transduced and measured by electrically sensing the resistance of a thermally sensitive device, such as a thermistor. Some other examples of parameters similarly amenable to measurement using the present invention are load (tension, weight, or pressure), pH, humidity, light intensity, or displacement (position).
Parameter measuring apparatuses can be used in a wide variety of areas in industry, as well as in consumer goods. For example, a temperature measuring apparatus may be used to measure the heat present in a motor, the heat present in a room, or the heat present in other systems, the output of which temperature measuring apparatus may be used in a feedback or other control circuit to control the heat source, and thereby maintain a constant temperature at the point of temperature measurement in the particular system involved.
Apparatuses for measurement of parameters are widely utilized and known in the prior art. Specifically, for example, temperature measuring apparatuses utilizing digital electronic circuitry, as in the present invention, are known in the art, as represented by Elfner et al., U.S. Pat. No. 4,413,325 for "An Apparatus for Determining the Temperature of an Asynchronous Motor"; Blieker, U.S. Pat. No. 4,455,095 for "A System for Measuring the Heat Energy (mission of Room Heating Elements"; Ikeda et al., U.S. Pat. No. 4,634,292 for "An Electronic Thermometer"; and Packard, et al., U.S. Pat.No. 4,642,785 for "A Cordless Electronic Thermometer". All of the above listed patents employ the use of an analog-to-digital conversion to convert an analog temperature sensor output to a form which is usable by digital electronic circuitry.
Wingate, U.S. Pat. No. 4,480,312 for "A Temperature Sensor/Controller System", uses a voltage-to-frequency converter to convert the analog output of a temperature sensor to a form usable by a microcomputer in digital electronic circuitry, as does the present invention.
The present invention is usable in a wide of variety of applications but was primarily developed for use in medical systems such as temperature measurement and control of heating pads, heating mattresses, cooling mattresses, and the like.
The various electronic components used in temperature measuring systems, such as temperature sensors, voltage or current sources, and the like, often may vary in their response as voltage applied to them varies, as the ambient temperature in which they operate varies, or for some other reason, such as excessive humidity or the like.
Generally speaking, precision in a measurement device is achievable at the expense of an increased cost of components used to construct the system. Many systems seek to provide a precision voltage or current source in order to insure that the response of the various components within the system is stable in the environment in which it is to be used. This is the approach taken in the Wingate invention. Wingate employs a precision voltage reference to insure that the supply voltage for his system is precisely controlled.
The present invention is designed to provide a precision parameter measuring apparatus, such as a temperature measuring apparatus, which is reliable in its operation to a high degree of accuracy over a wide variety of operating conditions which may be encountered in use. The present invention achieves such stable operation using stable passive reference components and microprocessor calculating capabilities, rather than relatively expensive stable signal source circuitry, to reliably and accurately measure and indicate temperature in spite of destabilizing influences such as variance of supply voltage, variance of ambient temperature, variance of ambient humidity, or the like.